Revive build Javadocs on JDK 10 and workaround bug (elastic#29173)

This commit reenables the Javadoc tasks on JDK 10. To reenable these
tasks, we have to workaround a bug in JDK 10 which trips on some deeply
nested anonymous classes that we have in the codebase (and are fine
as-is, this is not a problem with this code). The workaround is to
remove the compiled classes from the classpath. This has been reported
upstream and the workaround was suggested there (see the code comment).

Author: jasontedor

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -475,14 +475,18 @@ class BuildPlugin implements Plugin<Project> {
     }
 
     static void configureJavadoc(Project project) {
-        project.tasks.withType(Javadoc) {
-            executable = new File(project.compilerJavaHome, 'bin/javadoc')
+        // remove compiled classes from the Javadoc classpath: http://mail.openjdk.java.net/pipermail/javadoc-dev/2018-January/000400.html
+        final List<File> classes = new ArrayList<>()
+        project.tasks.withType(JavaCompile) { javaCompile ->
+            classes.add(javaCompile.destinationDir)
         }
-        configureJavadocJar(project)
-        if (project.compilerJavaVersion == JavaVersion.VERSION_1_10) {
-            project.tasks.withType(Javadoc) { it.enabled = false }
-            project.tasks.getByName('javadocJar').each { it.enabled = false }
+        project.tasks.withType(Javadoc) { javadoc ->
+            javadoc.executable = new File(project.compilerJavaHome, 'bin/javadoc')
+            javadoc.classpath = javadoc.getClasspath().filter { f ->
+                return classes.contains(f) == false
+            }
         }
+        configureJavadocJar(project)
     }
 
     /** Adds a javadocJar task to generate a jar containing javadocs. */